1. Technical Field
The present invention relates to a transmitting apparatus, a transmitting method, a transmission control program and a transmission control program storage medium, a receiving apparatus, a receiving method, a receiving control program and a receiving control program storage medium, and is suitably applied, for example, to a radio communication system using a spread spectrum system.
2. Background Art
In recent years, as an access control method in packet transmission or the like, a carrier sense multiple access (CSMA) system has been used. In such the CSMA system, when a certain terminal tries to perform transmission, power measurement is performed previously to detect whether another terminal is on transmitting or not (this is referred to as carrier sense), and when the receiving voltage is a predetermined value or less, it is judged that any other terminal is not on transmitting, and the transmission is performed so as to avoid simultaneous transmission.
By the way, in recent years, a wireless local area network (LAN) system applying a spread spectrum (SS) system has been becoming practically used. Furthermore, an ultra wide band (UWB) transmission system sending and receiving data by using an extremely-wide band of some GHz with pulse has been proposed for an application such as a personal area network (PAN).
In a direct spread (DS) system which being a kind of the SS system, at the sending side, the transmission is being made after the spread of the occupying band by multiplying an information signal with a random code series called a pseudo noise (PN) code, and at the receiving side, the information signal is being reproduced after the reverse spread by multiplying the received spread information with the PN code. The UWB transmission system is a system that the spread ratio of this information signal is enlarged to the utmost limit. The signal of the UWB transmission system has only an electric power of an equal level or less than a noise level in each frequency area, therefore, any communication system using the UWB transmission system has an advantage that it is comparatively easily compatible with a communication system using another system.
FIG. 9 shows an example of sending/receiving signals in transmission using the UWB system. An input information signal D100 is being multiplied with the spread code D101, and thus the spread spectrum is performed. Sometimes this multiplication by the spread code is omitted depending on the communication system.
The spread information signal D102 being subjected to the spread spectrum is modulated by an impulse signal (wavelet pulse) on the UWB system, and the signal is transmitted as a spread modulation signal D103. As the modulation technique used at this time, any of pulse position modulation (PPM), phase modulation, amplitude modulation, etc can be used. Since the impulse signal used in the UWB system, because of its an extremely-narrow pulse nature of less than 1 nsec, will use an extremely-wide band of some GHz as a frequency spectrum. Therefore, the spread modulation signal D103 only has such electric power at a noise level or less in each frequency area.
On the receiving side, a receiving signal D104 is mixed in noise, but it can be detected by calculating the correlation value of the receiving signal D104 with the impulse signal. Moreover, in many systems, by such signal spreading, many impulse signals are transmitted for each one bit of transmission information. Thereby, the receiving correlation value D106 of the impulse signal can be further integrated by the length of spread sequence (a correlation integrated value D107), thus, the signal detection can be further more easily attributed.
In FIG. 10, a structural embodiment of a radio communication terminal using the UWB system is shown. The radio communication terminal 100 is constructed by elements for sending 101 to 104 and elements for receiving 105 to 110; a sending/receiving timing control part 111; an RF part 112; and a central control part 113 for totally controlling the said radio communication terminal.
At the time of transmitting, the information source coding part 101 of the radio communication terminal 100 performs predetermined source coding processing on an information signal entered from the outside, and supplies this to a communication channel coding part 102. The communication channel coding part 102 performs predetermined channel coding processing on the information signal, and supplies this to a pulse generating part 104 via a sending buffer 103.
The pulse generating part 104 modulates the information signal by the impulse signal based on the control by the timing control part 111, and generates a modulation signal spread by the UWB system, and it is amplified by the RF part 112, and then is transmitted via an antenna 114.
On the other hand, at the time of receiving, the RF part 112 of the radio communication terminal 100 amplifies a receiving signal received via the antenna 114, and supplies this to a pulse correlation unit 105. The pulse correlation unit 105 calculates the correlation of the receiving signal with the impulse signal, and further integrates this correlation value by the number of pulses for one bit of transmission signal by an integrator 106, and outputs this.
An analog-to-digital conversion part 107 digitally converts the correlation value integrated value being outputted from the integrator 1-6, and supplies this to a channel decoding part 109 as a demodulation signal via a receiving buffer 108. The channel decoding part 109 performs channel decoding processing on the demodulation signal, and supplies this to an information source decoding part 110. The information source decoding part 110 performs source decoding processing on the demodulation signal, restores the information signal, and outputs this to the outside.
In the UWB transmission system as described above, since a signal is spread in a wide band and it is transmitted, the power density of the signal in each frequency area becomes the power density of noise or less. Therefore, in the case where carrier sense is performed on the UWB transmission system, the presence of a spread information signal can be determined only after the spread information signal is synchronized and back-spread. However, it takes a time to perform the above synchronization and back-spreading processing on the spread information signal. Therefore, in a system for performing intermittent communication as packet transmission, there has been a problem that it is hard to perform carrier sense. There has been another problem that the preamble for synchronization added to each packet for obtaining the synchronization becomes long.